On Sun, Mar 06, 2022 at 07:25:11PM -0700, Jens Axboe wrote: > On 3/6/22 7:20 PM, Ming Lei wrote: > > On Sun, Mar 06, 2022 at 06:48:15PM -0700, Jens Axboe wrote: > >> On 3/6/22 2:29 AM, Christoph Hellwig wrote: > >>>> +/* > >>>> + * Reuse ->bi_end_io as hlist head for storing all dm_io instances > >>>> + * associated with this bio, and this bio's bi_end_io has to be > >>>> + * stored in one of 'dm_io' instance first. > >>>> + */ > >>>> +static inline struct hlist_head *dm_get_bio_hlist_head(struct bio *bio) > >>>> +{ > >>>> + WARN_ON_ONCE(!(bio->bi_opf & REQ_DM_POLL_LIST)); > >>>> + > >>>> + return (struct hlist_head *)&bio->bi_end_io; > >>>> +} > >>> > >>> So this reuse is what I really hated. I still think we should be able > >>> to find space in the bio by creatively shifting fields around to just > >>> add the hlist there directly, which would remove the need for this > >>> override and more importantly the quite cumbersome saving and restoring > >>> of the end_io handler. > >> > >> If it's possible, then that would be preferable. But I don't think > >> that's going to be easy to do... > > > > I agree, now basically there isn't gap inside bio, so either adding one > > new field or reusing one existed field... > > There'd no amount of re-arranging that'll free up 8 bytes, that's just > not happening. I'm not a huge fan of growing struct bio for that, and > the oddity here is mostly (to me) that ->bi_end_io is the one overlayed. > That would usually belong to the owner of the bio. > > Maybe some commenting would help?
OK, ->bi_end_io is safe because it is only called until the bio is ended, so we can retrieve the list head and recover ->bi_end_io before polling. > Is bi_next available at this point? The same bio can be re-submitted to block layer because of splitting, and will be linked to current->bio_list[]. BTW, bio splitting can be very often for some dm target, that is why we don't ignore bio splitting for dm polling. Thanks, Ming -- dm-devel mailing list dm-devel@redhat.com https://listman.redhat.com/mailman/listinfo/dm-devel